The use of rapeseed oil in mobile hydraulic systems has become more widespread over recent years. This is because of concern about the environment in which the systems work and the perceived benefit of using such fluids. This article examines the major segments of the life cycle of mineral and rapeseed oil as used in mobile hydraulic systems, with case studies of a forestry harvester and a road sweeper. It shows that the systems running on rapeseed oil are not necessarily better for the environment. Many of the environmental issues examined in the study were affected more negatively by the use of rapeseed oil than mineral oil. The main exception to this was greenhouse gas emissions, which are consistently higher for systems using mineral oil because of the use of fossil resources. This study examines the production of the machinery, the oils, and their use throughout the machines' lives. The poor environmental performance of the rapeseed oil is due mainly to its poor performance in the field. This is because it does not respond as well to high pressure and temperature as mineral oil, causing it to need more frequent replacement during use. This, in turn, influences the definition of the functional unit used in the life-cycle assessment that was conducted. Also, the rapeseed oil has more corrosive qualities than the mineral oil, and more hydraulic components need replacing during the life of a machine running on rapeseed oil than one running on mineral oil.